To achieve sufficient performance in wireless communication systems, it is sometimes useful to characterize the wireless channel. For example, a transmitting entity may need an estimate of one or more channel parameters to perform spatial processing, precoding, or adaptive modulation and coding in order to transmit data to a receiving entity. The receiving entity may need an estimate of one or more channel parameters to properly demodulate transmitted signals in order to recover transmitted data.
Pilots may be inserted in a transmitted data stream to assist a receiving entity with various functions, including not only channel estimation but also timing and frequency offset acquisition as examples. A pilot typically includes one or more modulation symbols known to both the transmitting entity and the receiving entity that are transmitted in a known manner. Since a pilot represents overhead in a system, it is desirable to minimize the amount of system resources used to transmit pilots.
Conventional systems employ a fixed pilot structure that provides an adequate number and distribution of pilot symbols for most receiving entities under most channel conditions. However, the pilot structures may be inadequate for challenging channel conditions, and the pilot structures may waste system resources during more benign channel conditions. Thus, there is a need for techniques to better match pilot structures to channel conditions.